Field of the Invention
The present invention relates to inspection apparatus for measuring properties of a target structure on a substrate, and to methods of operating optical systems such as inspection apparatuses.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications US20100328655A1 and US2011069292A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.
In order to measure targets at different locations across a substrate, the known scatterometers include a positioning system for moving the substrate and a measurement optical system relative to one another. The relative movement may be achieved by moving the substrate while the optical system remains stationary, by moving the optical system while the substrate remains stationary, or by moving both the substrate and the optical system. In a known example, the substrate is moved in one dimension (for example, an X direction) while the optical system moves in another direction (for example, Y). In any of these systems, it is necessary for the moving parts to come to a standstill before an accurate measurement can be made. The settling time involved in this adds to the overall measurement time, and reduces the number of measurements that can be made in a given time interval.
The known scatterometers include an illumination system for forming a spot of radiation to be used in making measurements. The inspection apparatus includes an illumination system comprising one or more radiation sources and an illumination optical system (illumination optics) for the delivery of the illumination with the desired illumination parameters. In practice, it will be desired that the illumination system can switch between different wavelengths of illumination between measurements. In the following, the term ‘light’ will be used for convenience to refer the illuminating radiation, without implying any limitation to visible wavelengths. Different wavelengths of light experience different refractive indices in an optical system. Even though measures may be taken to reduce this so-called ‘chromatic aberration’, the measurements can be undesirably different at different wavelengths.